1. Field of the Invention
The present invention relates to dipole antennas, and more particularly to a method and structure for improving sidelobe performance of dipole array antennas.
2. Description of the Prior Art
Conventional dipole antenna elements such as shown in FIG. 1, comprise a length L of conductor sections 10 and 10' that are sometimes referred to hereinafter as dipole conductors, and are approximately one half of a free space wavelength (.lambda.) long including spacing G therebetween and extend substantially in the same plane parallel to and spaced above a conducting ground plane 11 by a distance H that is approximately 1/4.lambda.. The dipole conductors 10 and 10' are held in this position each by a respective support post portion 12 and 12' respectively, which are referred to hereinafter as posts. A member 13 centered between the posts serves as the RF feedline to one of the conductors such as 10'. The dipole conductors 10 and 10' are separated by the gap G, across which the RF drive voltage is applied. Each of the dipoles 10 and 10' together with their respective support post 12 and 12' have a predetermined width dimension that extends parallel to the ground plane 11, and is referred to as W, which may be approximately 0.0012.lambda.. Although the posts 12 and 12' are illustrated as metallic strips, they may be in the form of one half of a split tube, similar to the outer conductor of a coaxial cable. Many methods are used to supply a balanced voltage across the dipole conductors 10 and 10' and many of these methods involve transforming from an unbalanced type of transmission line such as a coaxial line or a strip transmission line, for example. Regardless of the method utilized, the objective of the drive voltage is to produce equal and opposite currents i.sub.a and i.sub.b in the two halves of such dipole conductors 10 and 10' as shown in FIG. 2 to produce a dominant (TEM) mode as represented by a vector E which radiates from the horizontal dipole conductors 10 and 10' as shown.
The conventional dipole element as described above is particularly advantageous when utilized in an array of many elements, because they inherently offer wide bandwidths, but have demonstrated unsatisfactory sidelobe performance. Where low sidelobes are desired, it is customary to utilize slotted waveguides arrays. Such slotted waveguide arrays, however, are limited in their instantaneous bandwidth.
Thus, it is desirable to be able to provide an array of dipole antenna elements that demonstrates the low sidelobe characteristics of slotted waveguide arrays, and yet maintain their wide bandwidth capability.